Pulse width modulation system for AC motor drive inverters

Abstract

A pulse width modulation system for AC motor drive inverters consists of a ROM for storing data of values of cos θ and cos(θ-π/3) with respect to a phase θ in the range of electrical angle of 60°, a programmable counter for counting the phase θ, an operational circuit for performing logical operations to evaluate each period of time for which a three phase AC motor is energized in accordance with each switching pattern, and a programmable timer for counting each period of time for which the motor is energized in accordance with each switching pattern. The motor is energized in accordance with each switching pattern for the period of time the data of which is gained at the operational circuit so that three phase output voltage resultant vectors form a circular locus.

What is claimed is: 
     
       1. A pulse width modulation system for AC motor drive inverters comprising: (a) first means for storing data of eight combinations of switching pattern signals representing combinations of on-off conditions of three switching elements;   (b) second means for storing data of a first time period ratio represented as cos θ corresponding to a phase θ in the range of from 0° to 60° of an electrical angle and data of a second time period ratio represented as cos (θ-π/3);   (c) third means for producing data of a triangular wave, the cycle of which is 2T where T represents a time period of the electrical angle of 30° or less;   (d) fourth means for sequentially obtaining data of three kinds of time periods by dividing the cycle T into three intervals at each cycle T by comparing the data of the triangular wave with the data of the first and second time period ratios while the first and second time period ratios change depending on θ in the range from 0° to 60° of the electrical angle; and   (e) fifth means for operating said fourth means repeatedly so that sequential formation of three predetermined switching patterns for time periods determined by the data of the three kinds of time periods produced sequentially is reiterated at 60° phase increments.